Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

15.3
Treatment of Metal Ions in Wastewater
15.3.1
Conventional Methods
A variety of methods are available to remove metal ions from water, which include ion
exchange, electrochemical reduction, adsorption, membrane filtration, chemical precipitation
and reverse osmosis [6,8,42,45,46]. The most common method cleaning up
the metal ion contaminated water is chemical treatment, by which metal ions are
precipitated using basic reagents such as hydroxides (e.g. lime) to form sludge, which
is then disposed off as a hazardous waste or, in some cases, reused or recycled.
Chemical treatment remains the favored approach because it is reliable, efficient and
reasonably inexpensive ($0.18–0.26/1000 l) [6]. However, the sludge thus created is
difficult and costly to handle or dispose off. For some metal ions with special characteristics,
large amounts of flocculating agents are required, resulting in the formation
of excessive metal sludge that requires high disposal costs [45]. For instance, to
remove 1 lb (0.45 kg) of copper from 1000 mg l 1 Cu 2+ solution, costs average $76.99/
1000 l of copper ion containing wastewater [39]. Over 80% of the cost is for disposal of
the hazardous sludge. Moreover, this technology may prove very costly if large volumes
of low metal concentration and high cleanup standards are involved.
15.3.2
Microbial Methods
15.3 Treatment of Metal Ions in Wastewaterj285
Bacteria, algae and fungi can remove metal ions from the external environment by
means of metabolism-dependent and metabolism-independent process to take up
and accumulate metals on the cell surface and inside the cells [13]. Microbial cells
can accumulate metal ions both in metabolism-dependent ways by precipitation,
redox reactions and ion transport systems and in metabolism-independent ways by
biosorption [13,47]. From toxicological perspective, microorganisms accumulate
metal ions on the cell surface and within the cell by their metabolic activities, such
as formation of metal sulfides by sulfate-reducing bacteria [48], oxidation of Fe 2+ to
Fe 3+ by iron-oxidizing bacteria [49] and transportation of metal ions into cytoplasm
by ion pumps; they are detoxified by complexing with siderophores or metallothionein
in cytoplasm [18,50]. Biosorption involves nonactive uptake of metal ions by
microbial biomass and includes physical adsorption, ion exchange, complexation,
precipitation, crystallization and diffusion [12,18]. Metallic cations are attracted to
negatively charged sites at the surface of the cells. The anionic ligands, which
participate in metal binding, include phosphoryl, carboxyl, sulfuydryl and hydroxyl
groups of membrane proteins [51].
The ability of microorganisms to remove metal has been utilized in metal recovery
or metal-laden wastewater remediation. The process involves living or dead cells in a
batch system or an immobilized cell system. Biosorption processes are more effective
in metal removal than conventional methods when the metal concentration in